This invention relates to a method for forming and bond (hermetic seal) between sapphire and glass. More particularly, the invention relates to a method for forming a bond between a sapphire faceplate and a glass envelope of a cathode ray tube.
Attempts to form a bond between sapphire and glass have been unsuccessful, especially with respect to the application of these material in a cathode ray tube as indicated above, because of the extreme mismatch of coefficients of linear expansion of the two materials. Sapphire has a coefficient of linear expansion of about 55 .times. 10.sup.-.sup.7 /.degree. C. and 0120 lead glass, a glass typically used in the manufacture of cathode ray tubes, has a coefficient of linear expansion of about 89 .times. 10.sup.-.sup.7 /.degree. C. In order for a cathode ray tube (CRT) to operate, very low absolute pressure (high vacuum) is required within the CRT.
Sapphire is a very desirable material for use in the faceplate of a CRT because of its heat dissipation capability. However, the low absolute pressure and high temperatures encountered in the CRT require that a metal or glass bond be formed between the sapphire and glass to satisfy the hermeticity requirement of the CRT application. Indium has been found to be a metal particularly suitable for effecting the necessary seal between the sapphire and glass because indium is a soft, pliable metal that yields, or, plastically deforms, under stress, thereby to minimize the accumulation of stress at the seal location. In addition, because indium has a melting point at about 152.degree. C., the sapphire and glass materials may be joined at or near this low temperature.
A sapphire CRT faceplate is a large, single-crystal element that has a critical temperature, between 1,000 and 3,000.degree. C., wherein extreme care must be taken to prevent its being cracked. This and other factors preclude the formation of direct sapphire-to-glass bonds, as can be obtained where both of the materials being sealed are glass. Thus, seals formed by conventional techniques employing the use of grit, flanges, grading, involving a gradual transition between dissimilar materials, and brazing are unsatisfactory for use with sapphire and glass. Also, specific prior art methods for forming bonds between dissimilar metals, such as those methods described in U.S. Pat. Nos. 3,590,467 to Chase et al, 3,798,746 to Alphonse et al and 3,747,173 to Lind, have proven unsatisfactory in connection with attempts to form a bond or hermetic seal between sapphire and glass for CRT applications.